SYMPOSIA...
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| Comparitive Colour Vision |
| Misha Vorobyev
- The University of Auckland, New Zealand
Ecology and Evolution of Colour Vision
Why different animals have different colour vision and why did colour vision evolve? More than a hundred years ago Grant Allen suggested that colour vision in insects, birds and primates evolved as an adaptation for foraging on colourful advertisements of plants - fruits and flowers. Although this view remains dominant in literature on evolution of colour vision, recent molecular-genetic studies of opsins indicate that well developed colour vision had appeared long before fruits and flowers evolved. The most sophisticated colour vision system is found in ancient marine crustaceans – mantis shrimps (Marshall, 1988, Nature 333: 557-560). These crustaceans have 16 spectral types of photoreceptors, 12 of which are used for colour vision. Lamprey, the closest relative of ancestors of all extant vertebrates, has four types of cone opsin genes, which gives it a potential for tetrachromatic colour vision (Collin et al., 2003, Curr Biol 13: R864-R865). The same four groups of cone visual pigments are retained in teleost fish, reptiles and birds. Hence, vertebrates could have had tetrachromatic colour vision before the ancestors of modern lampreys and other vertebrates diverged in the early Cambrian. Because angiosperm plants appeared much later during the Cretaceous period, colours of fruits and flowers probably evolved for pre-existing colour vision systems. Also, the colourful plumage of birds evolved when birds already had tetrachromatic colour vision. The most common modification of this ancient vertebrate tetrachromacy is the loss of visual pigments. For example, modern placental mammals are dichromats, because their tetrachromatic ancestors lost visual pigments as a consequence of nocturnal life-style. While the loss of cone visual pigments occurred many times, the duplication of cone visual pigments among terrestrial vertebrates has been described only in primates. Because fruits are important component of primate diet, this gene duplication leading to trichromacy can be explained as an adaptation to foraging on fruits, as Grant Allen suggested. Alternatively, primate trichromacy could have evolved as an adaptation for many visual tasks. Interestingly, recent molecular genetic studiers suggest that duplication of opsin coding appeared many times in also in teleost fishes. It remains uncertain whether the duplications of opsin genes in fishes can be explained as adaptations to fish habits and habitats. |
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Paul Martin - The University of Melbourne, Australia
Co-authors: Christopher Tailby, Brett Szmajda, Ulrike Grünert, Sammy CS Lee
Colour channels and colour selectivity in the subcortical visual system of primates
Most mammals show dichromatic ("red-green colour blind") colour vision, based on signals originating in short wavelength sensitive (S or "blue") cones and medium-long wavelength sensitive (ML) cones. We studied this primordial blue-yellow pathway for colour vision in marmoset monkeys, by measuring the responses of single neurones in the visual afferent pathway (lateral geniculate nucleus, LGN), and the connections of S cones in the retina. We found that S cone recipient neurones (blue-on and blue-off) show large receptive fields and are segregated to the koniocellular layers of the LGN. In trichromatic marmosets the red-green opponent cells show small receptive fields and are segregated to the parvocellular layers. Red-green signals are thus multiplexed with signals serving high-resolution spatial vision. We found the connections of most bipolar cells in the retina are biased against S cones. These data show there is functional isolation of S cone signals to colour pathways. Such isolation might serve to prevent the S cone array (which is maximally sensitive to out-of-focus wavelengths) from degrading the spatial resolution of ML cone recipient neurons. |
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Shoji Kawamura - The University of Tokoyo, Japan
Evolutionary diversification of visual opsin subtypes in fish and primates: Spectral differentiation, expression patterning and natural selection
Fish and primates are highly polymorphic in color vision among vertebrates, possibly reflecting their remarkably variable light environment. To study the evolution of color vision in fish and primates, we have focused on gene duplication and allelic differentiation of their opsin genes.
By using zebrafish and medaka as model fish, we have shown that gene duplications of opsins have occurred repeatedly during fish evolution, often accompanied by differentiation of their spectral sensitivity and spatiotemporal expression patterns in the retina. We have also shown that a similar regulatory mechanism has evolved independently in fish and primates in which a single regulatory region controls the array of duplicated opsin genes.
By examining allelic variation of the L/M opsin gene from fecal DNA for wild populations of New World monkeys in terms of nucleotide diversity and Tajima’s D test, we have shown that balancing natural selection has acted on the maintenance of color vision variation. Our behavioral observation has shown that dichromatic monkeys are more excellent in catching camouflaged insects and can be as good as trichromats in foraging fruits, implying that niche divergence or mutual benefits among different vision types may be the nature of the balancing selection supporting the vision variation.
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Justin Marshall - The University of Queensland, Australia
Co-authors: Tom Cronin, Mike Land, Roy Caldwell
Stomatopod Colour and Polarisation Visual Systems
Since the discovery of the unique colour and polarisation vision system in stomatopods 20 years ago, further investigations on these wonderful animals has unearthed much to get excited about. This presentation aims to give a whistle-stop summary tour of the highlights. This includes: a) Multiple colour filters in photoreceptors and dioptrics, b) 12 Spectral sensitivities in one animal from 300-720nm. c) Tunable colour vision, both between species and within a single species in response to visual ecology demands. d) Linear Polarisation vision in 2 wavelength bands. e) Circular polarisation vision - a new form of vision in the animal kingdom. f) Eye movements and the pre-invention of remote sensing. g) Colour and polarisation signals, h) Vision, sex and violence. |
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Shaun Collin - The University of Queensland, Australia
Co-authors: H. J. Bailes, N. S. Hart, W. L. Davies, A. E. O. Trezise, N. J. Marshall, D. M. Hunt
Colour vision in our vertebrate ancestors: evolutionary strategies for vision in water and for venturing onto land
Colour vision is an important strategy for the identification of conspecifics, preferred prey and reproductive partners in animals that live in either water or on land. However, little is known about the selection pressures driving the evolution of colour vision in vertebrates. Therefore, we decided to examine photoreception in two key species of vertebrates (the Australian lungfish, Neoceratodus forsteri and the southern hemisphere lamprey, Geotria australis). These two species represent two of the earliest stages in vertebrate evolution and have remained relatively unchanged for 400 and over 500 million years, respectively. Using a multidisciplinary approach, we have determined that the retina of both the Australian lungfish and the southern hemisphere lamprey possesses 5 morphological types of photoreceptors each housing a different visual pigment within their outer segment. Each photoreceptor type is maximally sensitive to a different region of the light spectrum, optimally tuned to absorb wavelengths between 375nm and 624nm, often with the aid of different spectral filtering mechanisms (localised in the ocular media and/or the inner segment). The retina of the lungfish is clearly duplex with a compromise between sensitivity and resolving power, while the retina of the lamprey appears to contain some photoreceptors that possess both rod and cone-like characteristics. Although it is difficult to pinpoint the origins of scotopic photoreception, it is clear that photopic colour vision arose very early in vertebrate evolution (approximately 530 mya). Although it is impossible to accurately assess the light environment in ancient times, paleontological evidence and recent spectroradiometric analyses of the light environment of these extant species, suggest that their ancestors may have lived in a shallow, well lit aquatic environment, where they could sample (and tune) a wide range of spectral signals. The complex chromatic sampling strategies of these early vertebrates has therefore provided a rich palate of adaptations, enabling subsequent species to infiltrate a large diversity of ecological niches, including the important progression from water to land.
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